Attempted fix for ZIM file polling

[audiodude]

Addresses #988.

Not sure if this fully fixes the issue, but an LLM generated e2e test seemed to indicate that it does.

The only substantial change here is to check for a .zim extension in the files key of the webhook payload and avoid marking the ZIM as ready if it's not found.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.79%. Comparing base (7c138d7) to head (2de8645).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1070      +/-   ##
==========================================
- Coverage   92.83%   92.79%   -0.04%     
==========================================
  Files          74       74              
  Lines        4325     4289      -36     
==========================================
- Hits         4015     3980      -35     
+ Misses        310      309       -1     

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